A wide code division multiple access (WCDMA) system of the 3-rd generation partnership project (3GPP) uses a total of 512 long pseudo-noise (PN) scrambling codes in order to identify base stations (BSs). As a scrambling code of a downlink channel, each BS uses a different long PN scrambling code.
When a user equipment (UE) is turned on, the UE performs system synchronization of an initial cell and acquires a long PN scrambling code identifier (ID) of the initial cell. Such a process is referred to as cell search. The initial cell is determined according to a location of the UE at a time when the UE is turned on. In general, the initial cell indicates a cell having strongest signal level which is measured by a downlink signal.
To facilitate the cell search, a WCDMA system divides 512 long PN scrambling codes into 64 code groups, and uses a downlink channel including a primary synchronization channel (P-SCH) and a secondary synchronization channel (S-SCH). The P-SCH is used to acquire slot synchronization. The S-SCH is used to acquire frame synchronization and a scrambling code group.
In general, cell search is classified into initial cell search, which is initially performed when the UE is turned on, and non-initial search which performs handover or neighbor cell measurement.
In the WCDMA system, the initial cell search is accomplished in three steps. In the first step, the UE acquires slot synchronization by using a primary synchronization signal (PSS) on the P-SCH. In the WCDMA system, a frame includes 15 slots, and each BS transmits the PSS in the frame. Herein, the same PSS is used for the 15 slots, and all BSs use the same PSS. The UE acquires the slot synchronization by using a matched filter suitable for the PSS. In the second step, a long PN scrambling code group and frame synchronization are acquired by using a secondary synchronization code (SSS) on the S-SCH. In the third step, by using a common pilot channel code correlator on the basis of the frame synchronization and the long PN scrambling code group, the UE detects a long PN scrambling code ID corresponding to a long PN scrambling code used by the initial cell. That is, since 8 long PN scrambling codes are mapped to one long PN scrambling code group, the UE computes correlation values of all of the 8 long PN scrambling codes belonging to a code group of the UE. On the basis of the computation result, the UE detects the long PN scrambling code ID of the initial cell.
Since the WCDMA system is an asynchronous system, only one PSS is used in the P-SCH. However, considering that a next generation wireless communication system has to support both synchronous and asynchronous modes, there is a need for using a plurality of PSSs.
An orthogonal frequency division multiplexing (OFDM) system capable of reducing an inter-symbol interference effect with low complexity is taken into consideration as a new system for replacing the existing WCDMA. In OFDM, data symbols, which are serially input, are converted into N parallel data symbols. Then, the data symbols are transmitted through N subcarriers. The subcarrier has orthogonality in frequency domain and experiences independent frequency selective fading. An orthogonal frequency division multiple access (OFDMA) scheme is an OFDM-based multiple access scheme.
An OFDM/OFDMA system is feasible to a synchronization error such as a frequency offset or a time offset. Moreover, since the PSS is a first detected signal in a condition whether the synchronization error exists, detection performance needs to be ensured. If the PSS detection is not achieved, synchronization is not attained, resulting in delay of network access.
Therefore, there is a need for a method capable of ensuring PSS detection performance according to radio resources allocated to the P-SCH.